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Oak Knolls Mutual Water Company

What EPA’s and California’s records show in this utility’s water: nitrate, arsenic and other regulated chemicals; lead and copper; and violations since 2016, with the certification that handles each result.

Serves
123 people
Connections
47
Water source
Groundwater
Based in
Lake Isabella, CA
County
Kern
Owner
Private
Hardness
Very hard
The ZIP code it serves

93285

The short answer

1 result is above EPA’s legal limit, and 2 more are above the health goal. The worst: Nitrate, above the legal limit. 6 health-based violations since 2016.

What handles it

What was found

Lead and copper: EPA SDWIS monitoring periods ending 2016 to 2025.

Nitrate, uranium, total trihalomethanes (TTHM), fluoride, haloacetic acids (HAA5), arsenic and chromium (total): California’s own results, samples taken from April 1, 2020 to June 24, 2026, with EPA’s 2012 to 2019 file for earlier years.

Above the legal limit1

Nitrate

Comes from the source

Above the legal limit

23ppm

Highest sample; found in 117 of 117

2026 average: 10.9 ppm

From 2024 to 2026, at least 25 samples were above EPA’s legal limit of 10 ppm, the highest 23 ppm in 2024. The next sample at the same point, within 30 days, kept the average above the limit for 12; one came back under and 12 have no follow-up sample on file within 30 days. In 2026, the latest year on file, its 18 samples averaged 10.9 ppm, above the limit. EPA recorded violations for nitrate at this utility in 2024 and 2025. Earlier, from 2012 to 2019, EPA’s file shows a highest sample of 9.0 ppm, in 2019.

What handles it

  • NSF/ANSI 58 look for “Nitrate/Nitrite Reduction”
  • NSF/ANSI 53 look for “Nitrate/Nitrite (30 ppm challenge)”

Above the health goal2

Uranium

Comes from the source

Above the health goal

10.1ppb

Highest yearly average, 2023

Averaged by year, the highest was 10.1 ppb, in 2023: below EPA’s legal limit of 30 ppb, but above its health goal of zero. Its highest single sample was 12.8 ppb. That is the only year on file. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 9.1 ppb, in 2012.

What handles it

Lead

Comes from pipes, often your own

Above the health goal

0.9ppb

90th percentile, 2023 to 2025

In the latest period, 9 in 10 homes sampled were at or below 0.9 ppb, below EPA’s action level of 15 ppb. The action level drops to 10 ppb on Nov. 1, 2027. EPA’s health goal for lead is zero.

What handles it

Below EPA’s limits3

Total trihalomethanes (TTHM)

Forms or is added in treatment

Below EPA’s limits

19ppb

Highest yearly average, 2020 to 2023

2023, one sample: 5.4 ppb

Averaged by year, the highest was 19 ppb, in 2020, below EPA’s legal limit of 80 ppb. In 2023, the latest year on file, its one sample was 5.4 ppb. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 49 ppb, in 2018.

What handles it

  • NSF/ANSI 53 look for “Trihalomethanes (TTHM) Reduction”
  • NSF/ANSI 58 look for “Trihalomethanes (TTHM) Reduction”

Fluoride

Forms or is added in treatment

Below EPA’s limits

0.5ppm

Highest yearly average, 2022 to 2025

Averaged by year, the highest was 0.5 ppm, in 2025, below EPA’s legal limit of 4.0 ppm. Its highest single sample was 0.53 ppm. That was also the latest year on file. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 0.56 ppm, in 2016.

What handles it

Haloacetic acids (HAA5)

Forms or is added in treatment

Below EPA’s limits

1.9ppb

Highest yearly average, 2020 to 2023

Averaged by year, the highest was 1.9 ppb, in 2023, below EPA’s legal limit of 60 ppb. That was also the latest year on file. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 9.0 ppb, in 2018.

What handles it

No certification claim covers this compound yet.

2 compounds tested and not foundBelow the reporting level in every sample. Not found is not the same as zero.
  • Arsenic
  • Chromium (total)

What handles it

A certification covers specific claims, not a whole standard. Look for these claims on the certifier’s listing, not the box. Wording varies a little: NSF writes “Lead Reduction,” WQA just “Lead.”

  • NSF/ANSI 53

    Claims: Nitrate/Nitrite (30 ppm challenge), Lead Reduction

    For: Nitrate, Lead

  • NSF/ANSI 58

    Claims: Nitrate/Nitrite Reduction, Uranium, Lead Reduction

    For: Nitrate, Uranium, Lead

    Other certifiers’ wording: Nitrate

At the tap, or for the whole house?

Lead and copper usually come from pipes and fixtures inside or near your home. A whole-house system sits where water enters, so it cannot catch what your own plumbing adds after it. For those, the filter that counts is the one at the tap. How whole-house systems compare.

Filters with those claims on a certifier’s listing

Disclosure: TapBuff earns a commission if you buy through some links on this page. It does not change which standard or filter we name. As an Amazon Associate, TapBuff earns from qualifying purchases. How TapBuff makes money

Compare five-year costs

The best reverse osmosis systems, ranked

The best water filters for lead, ranked

Hardness

Very hard, typically 330 mg/L (19.3 grains per gallon), across 3 sampling points, 2019 to 2025.

Hardness is calcium and magnesium. EPA sets no limit for it: it shows up as scale on fixtures and in water heaters, and as soap that won’t lather. USGS calls water above 60 mg/L moderately hard, above 120 hard and above 180 very hard.

Softeners compared at 19.3 grains per gallon, salt includedSoftener or salt-free conditioner?California water hardness, utility by utility

Violations since 2016

Contaminant or ruleTypePeriodMeasuredStatus
NitrateA sample was above the legal limit, for a contaminant judged one sample at a time.EPA: Maximum Contaminant Level Violation, Single SampleJan. 1, 2025 to March 31, 202511 MG/LResolved
Lead and Copper Rule RevisionsThe required inventory of lead service lines was not completed or submitted on time.EPA: LSL InventoryOct. 17, 2024—Unaddressed
NitrateA sample was above the legal limit, for a contaminant judged one sample at a time.EPA: Maximum Contaminant Level Violation, Single SampleOct. 1, 2024 to Dec. 31, 202413 MG/LResolved
NitrateA sample was above the legal limit, for a contaminant judged one sample at a time.EPA: Maximum Contaminant Level Violation, Single SampleJuly 1, 2024 to Sept. 30, 202412 MG/LResolved
NitrateA sample was above the legal limit, for a contaminant judged one sample at a time.EPA: Maximum Contaminant Level Violation, Single SampleApril 1, 2024 to June 30, 202423 MG/LArchived
NitrateA sample was above the legal limit, for a contaminant judged one sample at a time.EPA: Maximum Contaminant Level Violation, Single SampleJan. 1, 2024 to March 31, 202419 MG/LArchived
Health-based violations only. Resolved and archived mean the utility returned to compliance or the state closed the record. 1 monitoring or reporting violation not shown.

What this report can’t tell you

  • Everything else. Nitrate, uranium, total trihalomethanes (TTHM), fluoride, haloacetic acids (HAA5), arsenic and chromium (total) results here are California’s own from 2020 on, with EPA’s national file for 2012 to 2019. Oak Knolls Mutual Water Company’s own annual report, which it must send you every year, has the contaminants this page does not show.

  • Your own plumbing. Lead results are the 90th percentile across homes the utility samples. Your home’s service line, solder and fixtures decide yours. A certified lab test is the only way to know.

  • What it means for your health. This page reports what the records show and EPA’s limits and goals; it is not medical advice. If you are worried about a health effect, talk to a doctor or your local health department.

  • Anything after the latest data. SDWIS data is from 2026 Q3; UCMR 5 didn’t sample this utility; California’s results run from April 1, 2020 to June 24, 2026.

Testing your own tap

Oak Knolls Mutual Water Company’s results are for the water it sends out. What your own service line, solder and fixtures add shows up only in a test of your tap, by a lab California certifies.

The 3 labs California certifies closest to the area Oak Knolls Mutual Water Company serves, in a straight line. Call first: certification is test by test, and not every lab takes samples from homeowners.

  • Pace Analytical Services, LLC dba BC Laboratories, Inc.

    Bakersfield, CA · about 37 miles

    Certified for bacteria, nitrate, lead and copper, metals, minerals and other inorganics, organic chemicals, disinfection byproducts, radioactivity

    (661) 327-4911

  • Zalco Laboratories, Inc.

    Bakersfield, CA · about 37 miles

    Certified for bacteria, nitrate, metals, minerals and other inorganics, organic chemicals, disinfection byproducts

    (661) 395-0539

  • BSK Associates - Bakersfield Analytical Laboratory

    Bakersfield, CA · about 42 miles

    Certified for bacteria

    (661) 327-0671

All 145 certified labs for California

Sources

PWSID CA1500465. Rebuilt from EPA data on Oct. 18, 2026.

Towns it serves

Also in Kern County

All California utilities

What to do next